PB1785: DISCOVERY OF APPROVED LOW-TOXICITY AGENTS DISRUPTING ACUTE MYELOID LEUKEMIA STEM AND PROGENITOR CELLS
Authors: J. Meilstrup, C. Leitch, M. Kriegbaum, V. Andresen, B. T. Gjertsen, K. Wennerberg
Journal: HemaSphere (2022)

Background: Acute myeloid leukemia (AML) is a disease driven by leukemia-initiating cells (LIC) that resemble early myeloid progenitors. The standard treatment of AML is intensive chemotherapy (IC) along with stem cell transplantation. However, with the average age of AML diagnosis being approximately 70 years, many AML patients are ineligible to receive IC. Instead, elderly patients are primarily treated with low-toxicity chemotherapy, originally, hypomethylating or low-dose cytotoxic agents and more recently, venetoclax in combination with hypomethylating agents. These treatments, however, are generally inadequate to eradicate the LICs and the disease almost invariantly recurs with poor overall survival. As such, there is an ongoing need to identify novel options of higher efficacy and lower toxicity treatment for elderly AML patients. In this context, many generally safe non-hematologic drugs exhibit sporadic adverse effects on bone marrow function, including cytopenias and bone marrow aplasia. This suggests that these low toxicity drugs affect myeloid progenitor cells and potentially target LICs in AML. Aims: To explore whether low toxicity drugs with known adverse effects on bone marrow function may be able to target AML initiating cells, and hence suggest their potential to be repositioned for treatment of elderly AML patients ineligible for IC. Methods: Experiments were carried out on primary AML patient samples and flow cytometry was used to identify subpopulation specific responses. Both short (5 days) and long-term (up to 28 days) assays were performed to explore the effects of drug treatment on the AML growth and cell renewal capacity. Results: Seven AML patient samples were screened in a 5-day ex vivo assay using a library of 21 low toxicity approved drugs with known myeloid adverse effects. Flow cytometry analysis identified four drugs; chloramphenicol, mefloquine, phenylbutazone and pyrimethamine, that target AML cells and decreased viability in a dose-dependent manner. These four drugs presented with tendency of low toxicity by preferentially targeting the AML cells rather than lymphocytes in the patient samples. This was found to be statistically significant for two of four drugs (P<0.05). Single treatments of the drugs were tested in primary AML sample long-term cultures and showed significant inhibition of the sample expansion as compared to control. Sequential flow cytometric analyses of the AML subpopulations showed that the drugs targeted the least differentiated subpopulations, in which the LICs are expected to reside. Such observations would support the treatments ability to target cells at the top of the AML hierarchy, which subsequently result in a loss of regeneration capacity and slower diminish of the leukemic bulk cells. Next, four AML samples were screened to observe for synergistic interactions between the drugs. We identified chloramphenicol-phenylbutazone combination of interest for continued investigation in long-term culture and in vivo settings. Summary/Conclusion: There is a potential of non-hematologic low toxicity drugs to be repurposed for AML treatment. Approved drugs reported to cause bone marrow failure may have an impact on the hematopoietic progenitors and if the drugs likewise affect the stem and progenitor cells in leukemia this could be translated into a therapeutic effect. By investigating some of these drugs on primary AML patient samples, we observed that several could selectively target leukemic progenitor populations. Taken together, this emphasizes the importance for further investigation of repurposing drugs for novel treatments toward AML.